Defrost the fish pie. This will take an hour or two, depending on the temperature in your kitchen.
To make the white sauce, put the milk, flour and butter in a saucepan and whisk over a medium heat until it thickens slightly.
Mix in the lemon juice, salt, pepper and mustard.
Mix in the half the cheese and set aside to cool.
If you're making a family size fish pie, you can use the fish straight from the pack, but as these are mini pies for kids, use kitchen scissors to snip the fish into half inch pieces.
Divide the fish between eight pie moulds. You may get one or two more or less depending on the size of your moulds - use your judgement and don't over fill them.
Divide the peas between each mould.
Pour the cooled sauce into each mould, stopping about 1cm from the top.
Cook the potato in the microwave according to the pack instructions.
Top the pies with the potato and the remaining cheese.
Bake in the oven for 25 minutes until golden and bubbling. Enjoy!
Make sure the pie moulds are cooled enough to be safe before serving to children, or spoon the pie onto the plate.